1. Field of the Invention
The present invention relates to a method for forming an alignment layer of a liquid crystal display (LCD). More particularly, the present invention relates to a method for forming an alignment layer of an LCD capable of preventing Mura defects when the alignment layer is formed through an LC one drop fill (ODF) process.
2. Description of the Prior Art
As generally known in the art, an LCD can be fabricated in a compact size with light weight and low power consumption functions, so the LCD has been employed in various information appliances or video appliances, instead of a cathode ray tube (CRT). In particular, a thin film transistor (TFT) LCD can provide a large-sized display screen with a superior image quality corresponding to that of the CRT, so the TFT LCD has been spotlighted in notebook PC and monitor markets. In addition, it is expected that the LCD may make inroads into TV markets.
Such an LCD mainly includes a lower substrate (TFT array substrate) having a pixel electrode, an upper substrate (color filter substrate) having a common electrode and a liquid crystal layer aligned between the lower substrate and the upper substrate. In addition, an alignment layer, which is an organic polymer layer, is formed at an inner uppermost portion of each substrate in order to align liquid crystal in a predetermined direction and a polarizing plate is attached to an outer surface of each substrate. The organic polymer layer is subject to a rubbing process after the organic polymer layer has been coated on the substrate in order to allow the organic polymer layer to have an anisotropic characteristic.
In most cases, the liquid crystal layer is formed through a liquid injection process using osmotic pressure. However, as the size of the LCD becomes enlarged, it may frequently happen that the liquid crystal is not completely injected into the LCD through the liquid injection process, causing the defect of the LCD. In particular, since relatively long time is necessary to inject the liquid crystal between substrates, productivity of the LCD may be lowered if the liquid crystal layer is formed through the liquid injection process.
To solve the above problem, an LC one drop fill (ODF) process capable of simplifying manufacturing processes while improving productivity of the LCD has been currently suggested. According to the above LC ODF process, a predetermined amount of liquid crystal drops onto the lower substrate, and then the lower substrate is combined with an upper substrate under a vacuum atmosphere, thereby forming a liquid crystal layer.
Hereinafter, a method for fabricating an LCD by using a conventional ODF process will be briefly described.
First, a lower substrate and an upper substrate, which have undergone an array process and a color filter process, respectively, are prepared. Then, an alignment material is coated on the lower and upper substrates, respectively. After that, the alignment materials are subject to a rubbing process in such a manner that the liquid crystal acquires isotropic characteristics.
Then, after cleaning the lower and upper substrates, a sealing agent is coated on an edge part of the upper substrate. At this time, the sealing agent can also be coated on the lower substrate. After that, the lower substrate is introduced into a liquid crystal dispenser, and then a predetermined amount of liquid crystal drops onto on a predetermined portion of the lower substrate, which corresponds to a display region of the LCD.
After that, the lower and upper substrates are introduced into a vacuum chamber where the lower substrate is combined with the upper substrate due to vacuum pressure applied thereto and a gap formed between the lower and upper substrates is filled with the liquid crystal. Then, if the sealing agent applied to the edge of the upper substrate has been cured, the LC ODF process is completed.
However, as the size of the LCD becomes enlarged, there is necessity to provide an alignment layer having superior characteristics for minimizing defects derived from the ODF process. In addition, since it is difficult to form the alignment layer corresponding to the large-sized substrate, Mura defects may occur when forming the alignment layer through the LC ODF process, degrading reliability of articles.
In detail, according to a conventional method for forming the alignment layer, a mixing solution of organic polymer materials consisting of polyimide (PI) and polyamic acid (PAA) is coated on the substrate, and then the mixing solution is pre-cured at the temperature of about 80° C. in order to volatilize a solvent contained in the solution and to align the organic polymer materials. Subsequently, the mixing solution is completely cured at the temperature of about 180 to 230 80° C., thereby forming the alignment layer. Then, the alignment layer is mechanically rubbed by means of a rubbing roller.
The conventional method may not degrade the performance of the alignment layer if it is applied to a conventional liquid crystal cell process using a hot press process or a liquid crystal injection process. However, if the conventional process is applied to a conventional liquid crystal cell process using the ODF process, vertical and horizontal tensions are applied to the alignment layer while the ODF process is being carried out, so that an alignment order between surfaces of the liquid crystal and the alignment layer may be degraded. As a result, the Mura defect may occur in the alignment layer, lowering the image quality of the LCD.